1. Field of the Invention
The present invention relates to a detection method for detecting a horizontal error in a scanning type radar device mounted on a vehicle, and more particularly, to a detection method for detecting a horizontal error between a scanning center line and the traveling direction line of a vehicle during straight driving.
2. Description of the Related Art
Recent vehicles may carry a radar device to detect an object such as a vehicle ahead or other obstacles ahead in order to avoid collision. As disclosed in Japanese Kokai No. 61-162776, such radar device transmits a radar wave, e.g., a light wave, radio wave, or ultrasonic wave from a transmitter to the forwarding direction, receives the radar wave reflected by an obstacle at the receiver, and determines the distance between the obstacle and the vehicle based on the difference between the time of transmitting a radar wave and the time of receiving the reflected wave. Furthermore, it is possible to detect an obstacle and its direction in a rather wide range by using a radar device which scans by emitting radar waves from the right to the left.
The above-described scanning type radar device calculates the direction of an obstacle by using the line indicating a vehicle's traveling direction during straight driving where the steering angle is zero, as a reference line. Therefore, the radar device needs to be mounted onto a vehicle under the condition where a scanning center line of the radar device is coincident with the reference line. In this way, a vehicle's traveling direction during straight line driving gets coincident with the center line of the vehicle body (hereinafter referred to as a "vehicle-body center line"), and the scanning center line gets coincident with the vehicle's traveling direction.
However, a mounting error may occur when a scanning type radar is mounted onto a vehicle in an assembling plant. This mounting error is one of the causes generating horizontal errors of the scanning center line. This error may cause a detected distance and direction of an obstacle to be erroneous.
Furthermore, if the scanning center line is set to be coincident with the vehicle-body center line, when the diameter of left tires differs from that of right tires due to an imbalance of air pressure in each tire, the traveling direction line and vehicle body center line become not matched, as shown in FIG. 2. As described earlier, in the assembling plant, the radar is mounted onto a vehicle so that the scanning center line of the radar lies upon the vehicle-body center line where the steering handle of the vehicle is in a neutral position, and the diameters of both sides of tires are the same. However, after the mounting, if the diameter of one side of the tires becomes different from the other, the vehicle-body center line do not match with the scanning center line. This may cause an erroneous control in obstacle avoidance because the distance and direction of the obstacle is accurate only on the coordinate system of the radar where the neutral position of the steering handle is set to be the origin of the coordinate system.